The conventional clock transmission is usually realized by a constant bit rate stream. For example, the external clock of the telecommunication equipment is transmitted through the constant bit rate stream, such as a Synchronous Digital Hierarchy (SDH) signal, or a Synchronous Optical Network (SONET) signal, or a Plesiochronous Digital Hierarchy (PDH) signal.
With the development of data services, that a Time Division Multiplex (TDM) service is transmitted through a Packet-Switched Network (PSN) has become a new focus of the industry. In such an application, the clock of the TDM service should be transmitted over the PSN. Because the packet service is a burst non-constant bit rate stream and the PSN is an asynchronous network, how to transmit and restore a clock over the PSN is a problem in the application of transmitting the TDM service in the PSN.
When the TDM service is transmitted over the PSN, according to a conventional method for restoring the clock of the TDM service, the clock of the TDM service is restored by circuit simulation. FIG. 1 is a diagram illustrating the principle of a conventional method for restoring the clock of the TDM service. As shown in FIG. 1, data packets are sent using the clock of the TDM service at the sender, while at the receiver, the data packets are received through First In First Out (FIFO) and the clock of the TDM service is restored according to the frequency of the data packets received and the state of the FIFO. The process is described below.
The sender sends the data packets at a rate synchronized with the clock of the TDM service, and the data packets are transmitted to the receiver over the PSN. The receiver performs the physical layer PHY and Media Access Control (MAC) parsing for the data packets received. The data obtained through parsing are first stored in an FIFO buffer at a rate equal to the rate at which the data packets arrive, that is, equal to the frequency at which the sender sends the data packets in a normal condition.
The data in the FIFO is transmitted to the Client Equipment (CE) by using the clock restored locally. The local clock is generated through filtering by a phase-locked loop (PLL) according to the state of the FIFO, that is, the clock output of the PLL is adjusted by the filling degree of the FIFO. More specifically, the frequency of clock output of the PLL becomes slower if the FIFO buffer is becoming empty; on the contrary, the frequency of clock output of the PLL becomes faster if the FIFO buffer tends to overflow.
In the PSN, there are some impairments, e.g., packet transfer delay and delay variation, packet loss, packet error, etc. According to the conventional method for restoring the clock of the TDM service, these impairments make the state of the water mark of the FIFO always fluctuate at random greatly, which has a detrimental effect on the jitter and wander quality of the restored clock.
In general, an internal clock of telecommunication equipment is distributed through a square wave signal such as the Transistor-Transistor Logic (TTL) level signal or the Low Voltage Differential Signal (LVDS) level signal. Moreover, there is only one system clock in a synchronization network element and the system clock is distributed to all the line cards from the timing card through the backboard. However, in the Optical Transport Network (OTN) switched system, the clock must be switched together with the service. Thus, a clock switched matrix should be established. It is however difficult to implement the clock switching because the frequency of the OTN clock is very high. And the cost of the clock switched matrix is also very high.